In backing the code Mr Bridges resisted calls from the AA and Consumer to regulate the industry.

But he has warned the industry that regulation remains an option if behaviour is not improved.

When announcing the code last month Mr Bridges said that similar moves seemed to have worked well in other countries, including Britain.

But British AA spokesman Andrew Howard told the Herald that self-regulation in the industry was a total failure. Last month Royal Assent was given to new legislation which would outlaw clamping on private property.

A previous voluntary code of conduct and licensing system had failed to clean up industry behaviour, he said.

"A voluntary code of conduct is all very well for the reputable, but the trouble is it's an industry which is too easy to get into and be disreputable.

"You're stuck by the fact there's very little you can do if someone doesn't abide by the code of conduct - people still have to pay the money."

More than 80 people have contacted the Herald with complaints against wheel clamping companies after a series of articles this year. In some cases, agents sneaked up behind cars to clamp a wheel while people were still inside, and others said they were abused when they could not pay an on-the-spot fine.

NZ Wheel Clamping, the company at the centre of most complaints, says it already operates by a code of conduct and has an "independent" appeals process.

Requests for refunds are passed from call centre staff to a company employee, who works in an area separate to other employees.

People can challenge that decision in a Disputes Tribunal, although a filing fee of $36.30 must be paid regardless of outcome.

Consumer advocates say spending more money and time to take a case to a tribunal is unrealistic for many people.

Ministry of Consumer Affairs briefing documents provided to Mr Bridges last month, released under the Official Information Act, recognise the limitations of relying on the tribunal to sort disputes.

"While the filing fees are low, when combined with the time and effort ... the consumer may not feel it is worth the bother."

Mr Howard said the British AA recognised landowners needed a way to stop people parking illegally on their property.

It is pushing for an amendment which would see private parking tickets subject to an independent appeals system, paid for by the parking companies.

Mark Stockdale, principal adviser on regulations for the AA, said the smaller size of the industry here meant regulation could work.

"Ultimately we'll be calling for regulation, but in the absence of that, maybe banning clamping is an option that we would need to consider."